The present invention relates to a polytetrafluoroethylene granular powder containing no filler and a preparation process thereof.
Hitherto many proposals with respect to a process for preparing a granular powder by finely-pulverizing a polytetrafluoroethylene (PTFE) powder prepared through suspension polymerization to an average particle size of less than 100 xcexcm and then granulating have been made. For example, JP-B-44-22619 discloses a process for stirring and granulating a PTFE powder in an aqueous medium of 30xc2x0 to 150xc2x0 C. containing a water-insoluble organic liquid having a boiling point of 30xc2x0 to 150xc2x0 C.
In order to improve the process disclosed in the above-mentioned patent publication, JP-B-57-15128 discloses a process in which an equipment having mechanism for pulverizing a PTFE powder is employed.
As a process for granulating by using water only, for example, JP-B-43-8611 discloses a process for granulating in which a PTFE powder is stirred in water of 40xc2x0 to 90xc2x0 C., JP-B-47-3187 discloses a process for granulating in which a PTFE powder is stirred in an aqueous medium of not less than 40xc2x0 C., and further JP-A-3-259926 discloses a process for granulating in which a PTFE powder is stirred in an aqueous medium in combination use of pulverizing mechanism.
However, in such granulation processes, electrostatic charging occurs on the PTFE powder due to static electricity. The electrostatically charged PTFE powder adheres to not only a die mold but also a hopper and feeder due to static electricity at the time of molding, which results in lowering of flowability. Also, there is a tendency that the finer the powder is, the more the flowability and apparent density are lowered. From these points of view, the PTFE powder obtained by the above-mentioned processes is not satisfactory.
Also a granulation process with a roughly pulverized PTFE powder having an average particle size of not less than 100 xcexcm has been proposed. As the process using a roughly pulverized powder, for example, JIP-A-3-259925 discloses a process for granulating in which a PTFE powder of 420 xcexcm particle size is stirred in an aqueous medium of 60xc2x0 to 100xc2x0 C. in combination use of pulverizing mechanism.
Even by the process disclosed in the above-mentioned patent publication, the obtained PTFE granular powder does not sufficiently satisfy physical properties of a molded article such as tensile strength, and for commercial production, a further step such as a gelling and pulverizing step is required. Also since a molded article obtained by molding the granular powder has a low break down voltage, it cannot be used for applications requiring a high break down voltage. Further as mentioned above, that process requires the pulverizing mechanism.
The present inventors have made intensive studies in view of the above-mentioned problems, and as a result, have found that those problems can be solved by granulating a finely pulverized PTFE powder in the presence of an organic liquid which forms liquid-liquid interface with water, and a nonionic surfactant and/or an anionic surfactant with stirring in water. Also the present inventors have found that the above-mentioned problems can be solved by granulating with stirring in the same manner as above even by using a roughly pulverized PTFE powder.
Namely, an object of the present invention is to provide a PTFE granular powder containing no filler and having a large apparent density, a small average particle size, a narrow particle size distribution, a small electrostatic charge, a small angle of repose and good physical properties such as powder flowability, and a process for preparation thereof. Particularly in case where an average particle size of a PTFE powder as a raw material is small, the obtained PTFE granular powder gives molded products having excellent physical properties such as elongation and surface smoothness, a low surface roughness (smoothness), a large break down voltage and a high degree of whiteness (Z value).
The present invention relates to a process for preparing a PTFE granular powder containing no filler, which is characterized in that a finely pulverized PTFE powder prepared through suspension polymerization and having an average particle size of not less than 10 xcexcm and less than 100 xcexcm or a roughly pulverized PTFE powder prepared by suspension polymerization and having an average particle size of not less than 100 xcexcm and not more than 300 xcexcm is granulated with stirring in water in the presence of an organic liquid which forms liquid-liquid interface with water, and a nonionic and/or anionic surfactant.
In the above process, it is preferable that the nonionic surfactant is one having a hydrophobic segment comprising a poly(oxyalkylene) unit having 3 or 4 carbon atoms and a hydrophilic segment comprising a poly(oxyethylene) unit.
Also it is preferable that the anionic surfactant is a fluorine-containing surfactant having a perfluoroalkyl group or a perchlorofluoroalkyl group as a hydrophobic group.
It is preferable that an amount of the surfactant is from 0.01 to 10% by weight based on the PTFE powder.
Also as PTFE, in addition to tetrafluoroethylene homopolymer, there can be used a modified PTFE obtained by copolymerizing 99 to 99.999% by mole of tetrafluoroethylene with 1 to 0.001% by mole of perfluoro(vinyl ether).
Further the present invention relates to a PTFE granular powder containing no filler prepared by the above-mentioned process, in which an apparent density of the granular powder is not less than 0.6 g/cm3, particularly not less than 0.7 g/cm3 when a finely pulverized powder having an average particle size of not less than 10 xcexcm and less than 100 xcexcm is used as a raw PTFE powder.
As the granular powder, it is preferable that a flowability, angle of repose, average particle size and electrostatic charge of the granular powder which are measured by the methods mentioned hereinafter are not less than 6 times, not more. than 40 degrees, not more than 500 xcexcm and not more than 50 V, respectively.
Also as the granular powder, preferable are one which gives a molded article having a break down voltage of not less than 5 kV in case of a 0.1 mm thick skived sheet, one which gives a molded article having a surface roughness of not more than 4.0 xcexcm and one which gives a molded article having a whiteness (Z value) of not less than 95, particularly not less than 100.
Particularly preferable are:
a PTFE granular powder containing no filler which has an apparent density of not less than 0.7 g/cm3 and less than 0.8 g/cm3, an average particle size of not less thank 250 xcexcm and less than 500 xcexcm, an angle of repose of not more than 38 degrees and an electrostatic charge of not more than 10 V and gives a molded article having a whiteness (Z value) of not less than 95, preferably not less than 100, a break down voltage of not less than 7 kV in case of a 0.1 mm thick skived sheet and a surface roughness of not more than 2.5 xcexcm;
a PTFE granular powder containing no filler which has an apparent density of not less than 0.8 g/cm3 and less than 0.9 g/cm3, an average particle size of not less than 250 xcexcm and less than 500 xcexcm, an angle of repose of not more than 38 degrees and an electrostatic charge of not more than 10 V and gives a molded article having a whiteness, (Z value) of not less than 95, preferably not less than 100, a break down voltage of not less than 6 kV in case of a 0.1 mm thick skived sheet and a surface roughness of not more than 3.0 xcexcm;
a PTFE granular powder containing no filler which has an apparent density of not less than 0.9 g/cm3 and less than 1.0 g/cm3, an average particle size of not less than 250 xcexcm and less than 500 xcexcm, an angle of repose of not more than 37 degrees and an electrostatic charge of not more than 10 V and gives a molded article having a whiteness (Z value) of not less than 95, preferably not less than 100, a break down voltage of not less than 5 kV in case of a 0.1 mm thick skived sheet and a surface roughness of not more than 3.0 xcexcm;
a PTFE granular powder containing no filler which has an apparent density of not less than 0.7 g/cm3 and less than 0.9 g/cm3, an average particle size of not less than 150 xcexcm and less than 250 xcexcm, an angle of repose of not more than 38 degrees and an electrostatic charge of not more than 10 V and gives a molded article having a whiteness (Z value) of not less than 95, preferably not less than 100, a break down voltage of not less than 7 kV in case of a 0.1 mm thick skived sheet and a surface roughness of not more than 2.5 xcexcm; and
a PTFE granular powder containing no filler which has an apparent density of not less than 0.9 g/cm3 and less than 1.0 g/cm3, an average particle size of not less than, 150 xcexcm and less than 250 xcexcm, an angle of repose of not more than 37 degrees and an electrostatic charge of not more than 10 V and gives a molded article having a whiteness (Z value) of not less than 95, preferably not less than 100, a break down voltage of not less than 7 kV in case of a 0.1 mm thick skived sheet and a surface roughness of not more than 3.0 xcexcm.
In general with respect to the PTFE granular powder, the higher the apparent density is and the larger the average particle size is, the more the physical properties relating to handling thereof (angle of repose, flowability, electrostatic charge property, etc.) are enhanced. On the contrary, there is a tendency that the physical properties of the obtained molded article (mechanical strength, elongation, break down voltage, surface roughness, etc.) are lowered.
Though the PTFE granular powder containing no filler of the present invention has a novel excellent handling property and gives a molded article having novel excellent physical properties, in order to identify and evaluate the novel PTFE granular powder of the present invention, it is necessary to compare with granular powders having apparent density and average particle size within the same range, and thus fair evaluation can be obtained.
Therefore, the PTFE granular powders of the present invention are herein identified with apparent density and average particle size thereof.